Optical fibers find widespread use in acoustic sensors (Kirkendall & Dandridge, 2004, J. Phys. D: Appl. Phys. 37:R197-R216). Polymeric coatings over optical fibers are known to increase the acoustic sensitivity of the fiber (Hughes & Jarzynski, 1980, Applied Optics, 19(1):98-107; Giallorenzi et al., 1982, IEEE J Quantum Engineering, QE18, pp 626-665). Further enhancement of sensitivity has been demonstrated through the use of air-included polymer (AIP) coatings (Cole et al., 2002, 15th Optical Fiber Sensors Conf. (Portland Oreg.) vol 15 IEEE Catalog 02EX533). AIP coatings, however, include air bubbles that collapse as operating pressures increase which significantly reduces the sensitivity enhancement. AIP coatings, therefore, are not suitable for high pressure applications.
Many fiber optic based sensors are required for applications that entail high temperature and/or high pressure operating conditions, such as downhole sensors in the oil and gas industry, deep water seismic exploration, nuclear reactor performance and safety monitoring, earthquake monitoring and localization, earthquake precursor alarm systems, detection of nuclear weapon test and/or treaty violations.
Commercially available electrical downhole sensor systems (without use of cooling or temperature isolation devices) are limited to use in downhole applications at temperatures of 110° C. or below. Various solutions for permitting use of optical fibers at high temperatures have been proposed, including commercially available coated and/or jacketed fibers (for example, from OFS (Specialty Photonics) and Fiberguide Industries).
U.S. Pat. No. 6,915,056 describes a buffer tube for optical fibers that is easily strippable and will not be subject to melting or sticking to other buffer tubes during the cable manufacturing process. The buffer tubes comprise a blend of at least 40% by weight of a copolyether elastomer, at least 10% by weight of an amorphous thermoplastic polymer and at least 10% by weight of a rubbery modifier. Preferred copolyether elastomers include Hytrel® thermoplastic elastomers, particularly Hytrel® grade G5544.
U.S. patent application Ser. No. 13/414,157 (US 2012/0227504) provides an alternative to coated fibers in the form of a sensor array suitable for underwater use that comprises a cladded core, a polymer layer disposed on the cladded core, an optical fiber wound around the polymer layer and a second polymer layer disposed over the initial polymer layer and the optical fiber. Exemplary polymers described are polyether urethanes and polyester urethanes.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.